There have been countless attempts at manufacturing a foam by mixing starch with another plant-derived material or with a plastic resin that would pose no danger of emitting dioxins if incinerated after use, for example, and at utilizing such foams as thermal insulation materials or as cushioning materials or other such shipping and packaging materials, and a number of methods for manufacturing foams have been disclosed. Nevertheless, based on an examination of this disclosed information, it appears that no known technique has been established relating to the problem the present invention is intended to solve. To clarify the problem the present invention is intended to solve, general concepts, beliefs, related examples, and so forth were examined in the technological field to which the present invention is related.
It is commonly held that when steam or the like is used as a foaming gas, the material is discharged under pressure from an extrusion molding machine, and reduced-pressure foaming is performed, for instance, a polymer raw material that becomes fluid when the temperature is raised will flow as shown in FIG. 6A, with symmetrical flow lines around the center line in the drawing, when the pressure is relatively low, and as the pressure is gradually raised, slip is observed between the material and the walls. At even higher pressures the flow lines begin to be disarrayed, the material is extruded in a spiral shape, and this results in a lack of uniformity in the material. What happens here is called stick and flow, in which the flow of the material comes to a halt (sticking), after which the material starts sliding again (slipping). When this occurs, complete and intermittent slipping over the entire surface of the slip cone shown in FIG. 6B has been reported for high density polyethylene and other such polymer materials, and a similar phenomenon is seen not just with polymer compounds but also in the extrusion of bismuth-tellurium and other such metal materials that are thermoelectric semiconductors, for example. This is manifested as cracks in the extruded semiconductor. Various apparatus improvements have been attempted from a mechanical aspect in an effort to eliminate this problem, but the fact is that the fluid properties of polymer compounds are still not fully understood by researchers.
The materials that are the subject of the present invention are polymer compounds, and it is necessary to understand this phenomenon. Since the present invention involves a plurality of polymer compounds, however, it is uncertain whether the phenomenon witnessed with a single compound such as polyethylene would be the same, and the problem is even more complicated because we are dealing with a mixture in which the temperature dependency of viscosity and the thermal deformation temperature characteristics vary with the different raw material polymer compounds, which probably makes it even more difficult to understand the situation in terms of fluid dynamics or tribology.
The present invention relates to a field of technology in which such scientific understanding is not necessarily perfect, but at the very least, of course, we must know something about the material characteristics of the raw material polymer compounds being used. Let us now give a brief summary of the characteristics of a polypropylene resin, as an example of a plastic resin, and starch, which are believed to be the main contributors to fluidity out of the polymer compound materials used in the present invention.
Starch is usually in the form of macromolecular spherulites of a mixture of two homologues with different structures, amylose and amylopectin. The size thereof can vary greatly with the type of starch, but the diameter is said to range from 1 to 20 microns, and the molecular weight up to a few thousand. Amylose is a linear polymer to which D-glucose units are connected by α-1,4 linkages, whereas amylopectin is a branched polymer that includes α-1,6 linkages in addition to α-1,4 linkages, with branching occurring at these sites and the branches connected again with α-1,4 linkages.
When starch granules are suspended in water and heated to a certain temperature or higher, the starch granules undergo irreversible swelling and amylose is eluted. This process is called agglutination, and this temperature is referred to as the agglutination temperature. FIG. 7 shows published heating and viscosity curves for various kinds of starch (amylograms). The agglutination temperature ranges from 60 to 90° C., and cornstarch is what is cited in the embodiments and so forth of the present invention. FIG. 8 also gives published information, and is a comparison of the viscosity behavior of ordinary wheat resulting from agglutination, by variety of glutinous wheat (which has a relatively high amylopectin content), versus that of cornstarch.
For this comparison, the temperature of each starch was raised from 34° C. to 94° C. at a rate of 5° C. per minute, after which the starch was cooled at the same rate of 5° C. per minute and its viscosity was measured. Of these varieties, Norin No. 61 had the same amylose content of 31.8% as ordinary wheat, and its agglutination temperature was close to that of wheat as shown in FIG. 8. The waxy cornstarch used as a control also has a relatively low agglutination temperature, similar to what is shown in FIG. 7. The purpose of providing FIGS. 7 and 8 here is to show the agglutination temperature of the starch that is one of the polymer compound materials used in the present invention in relation to the fluidity of these materials, and demonstrate that there is a rapid increase in viscosity as soon as the agglutination temperature is attained.
The polypropylene resin will now be discussed. Polypropylene is a crystalline thermoplastic resin made by the polymerization of propylene (C3H6), and is hard and tough, with good resistance to moisture, oils, and solvents. Its heat resistance temperature is said to be 170° C. or lower. Also, there is almost no possibility that a linear polymer compound will undergo cyclization during combustion, and these resins, along with polyethylene and the like, are called polyolefin resins and said to be resins that do not produce dioxins or other harmful substances.
According to disclosed reference materials from polypropylene manufacturers and so forth, the thermal deformation temperature of polypropylene varies somewhat with the structure of the compound, but is said to be roughly 100 to 120° C. FIG. 9 shows an example of the elongation viscosity of the linear (without branches) polypropylene resin used in the examples and so forth of the present invention, and it can be seen that the elongation viscosity at 180° C. quickly disappears in just a few seconds. The reason a foam of a high expansion ratio cannot be produced from a linear polypropylene resin is said to be that the elongation viscosity drops off so quickly and the melt tension is low, and most polypropylene manufacturers produce polypropylene resins having a branched structure by subjecting a linear polypropylene to a special process, and then make a foam with a relatively high expansion ratio from this polypropylene resin with improved elongation viscosity and melt tension. Higher cost is at the present time inevitable with a branched polypropylene because a linear polypropylene must be subjected to a post-treatment step such as electron beam irradiation.
When an attempt is made to produce a foam by mixing and melting a starch and a plastic resin such as polypropylene, the difference between the temperature dependence of the viscosity behavior and the melt temperature of the polymer compounds results in complex fluidity of the mixed materials in the mixing, melting, and extrusion steps during manufacture, and it is not hard to imagine the effect that this has on discharge uniformity. Japanese Laid-Open Patent Application H9-111029 discusses a method for manufacturing a loose polypropylene foam by mixing a polypropylene resin and a starch-based additive into a plant-derived foaming agent such as nonfat powdered milk or bean-curd lees, putting this raw material mixture into an extruder comprising a high-temperature heated cylinder, and feeding in water. This publication also discusses a method in which a plastic component, plant-derived foaming agent, and starch-based additive are simply mixed.
The inventors of the present invention conducted manufacturing tests in which they used an actual foam manufacturing apparatus similar to that in the disclosed information, used as their raw materials a plastic resin, starch, and pulverized waste paper (as the plant-derived fibrous material), and varied the amount of feed water (the foaming agent), the mixing time and temperature in the manufacturing process, the conditions of the material discharge orifice, and so forth. As a result, with a simple mixture of raw materials as in the disclosed information, perhaps because variance in fluidity could not be suppressed, there was considerably fluctuation in the discharge amount per hour, and there was also variance in the foaming state, so it was found that a foam of stable quality could not be manufactured. The specific manufacturing method and steps here are illustrated in FIG. 10.
Pulverized paper (such as waste paper) was used as the plant-derived fibrous material, but first the material was finely pulverized in a mixer/cutter type of pulverizer and then in a ball mill type of pulverizer, this pulverized material was sifted through an industrial sieve to obtain a waste paper powder with particles averaging 20 to 30 microns in size, and this powder was used as the waste paper raw material. This waste paper raw material was then mixed with a polypropylene resin to obtain a raw material mixture, to which cornstarch was further added and mixed, and these components were put in a mixing machine and mixed. The sequence of this mixing was also varied so that the waste paper raw material was first mixed with the cornstarch to obtain a raw material mixture, to which the polypropylene resin was added and mixed in a mixing machine. The mixing was also performed at various blend ratios, including those given in the disclosed information. This kneaded raw material was put into,a biaxial extruder and foamed. Specifically, the biaxial extruder shown in FIG. 11 comprises a raw material inlet 1, a water inlet 2, cylinders C1 to C5, heaters 3 attached to the various cylinders, a raw material extrusion die (spinneret) 4, and an extrusion opening 5. In manufacturing tests, the settings of the cylinder and extrusion die temperatures was suitably varied. The test was conducted with the water supply varied between 10 and 30 liters per hour.
To evaluate the foam state and foam variance, a string-like piece of foam extruded and foamed from an extrusion die with a diameter of 2 mm was cut up by an automatic cutter set to a specific rotating speed, 50 pieces were randomly sampled from among the total of 4200 pieces of cut foam produced in one minute, whose combined length was 120 m, the length and diameter of the cut foam pieces were measured, and the extent of foaming was visually evaluated on a ten-point scale.
The mixing ratios of the materials for which the results are shown in FIG. 14 were 25% (weight ratio) pulverized waste paper, 30% starch, and 45% polypropylene resin.
As indicated by these results, the foam manufactured by the mixing process described above exhibited considerable variance in all measurement categories. The length of cushioning material cut at regular time intervals varied from a minimum of 1.9 cm to a maximum of 8.5 cm. The most serious problem was variance in the extent of foaming. The ten-point scale assumes 10 to be the ideal foaming state, but because there was variance in the foaming, the cushioning material varied from thin to thick. This indicates nothing other than a change over time in the fluidity of the raw material mixture during mixing and melting inside the cylinders. This is similar to the stick and flow phenomenon described above, but it is unclear whether it occurred because a slip cone was formed, or just because of local variance in the fluidity of the molten mixture.
Admixed in this molten mixture was pulverized waste paper, whose characteristics remained almost unchanged over the temperature range in this process. This pulverized paper consists of a fibrous material with a microscopic length of anywhere from a few microns to about 100 microns, which is stirred and mixed in the biaxial extruder, and it seems that nuclear cells that will serve as the nuclei for cells are produced here and there in an agitated fluid in which polymer compounds of different viscosity are admixed. In this situation it is doubtful whether a nice slip cone is being formed as in the above-mentioned reports on polyethylene resins. On the contrary, it seems likely that this nucleus formation is occurring as a result of local variance in the fluidity of the molten mixture.
At this point we will briefly discuss the correlation between the viscosity of the above-mentioned starch and polypropylene resin and the fluidity of the mixture of polymer compounds, since such a discussion will probably further clarify the points involved in the problems which the present invention is intended to solve and the means used to solve the problems. A capillary type of viscosity measurement apparatus is the most commonly used in the measurement of the viscosity of polymers. A sample is extruded from a capillary tube by applying pressure, and the viscosity is measured by measuring the difference in the applied pressure at a constant flow rate per unit of time. We will let L be the length of the capillary tube, and r be its radius. The change over time in the amount of sample flowing out of the capillary tube is found for when pressure is applied such that the sample contained in the reservoir flows at a specific rate, and this is termed Q (cm3/sec). A relationship ofγ=4Q/πr3
exists between the shear rate γ (sec−1) and the sample flowing through the capillary tube, and the shear stress τ (N/cm2) on the inner walls of the capillary tube is expressed as τ=rP/2L. The viscosity μ (N·sec/cm2) is defined from the shear rate γ and the shear stress τ as:μ=τ/γ.
If we assume that a starch and a polypropylene resin are mixed and the thermal characteristics of each are independently manifested, the intrinsic viscosity should be exhibited in the starch at the agglutination temperature and in the polypropylene resin at the thermal deformation temperature, with the viscosity subsequently varying according to temperature changes. For instance, if the agglutination temperature of the starch is reached before the thermal deformation temperature is reached, the viscosity of the starch increases suddenly, and the entire system is governed by the viscosity of the starch. If the thermal deformation temperature of the polypropylene resin is never reached, the polypropylene might have some kind of effect as an admixture, but the viscosity μ of the system will be relatively stable. In other words, this is a state in which the fluidity of the system is governed by the viscosity of the starch, even though the system is a mixture.
However, this never happens for the purposes of manufacturing a foam. Still, if the temperature continued to rise, it would eventually reach the thermal deformation temperature of the polypropylene resin, resulting in a fluid having both the viscosity of polypropylene and the viscosity of the starch, which varies with time and temperature. It is very difficult to know what the mixing state is in a mixed fluid such as this. Measurement was made at a certain specific temperature with the above-mentioned viscosity measurement method, but since the mixing is not completely uniform in a mixing state of different viscosities, the local shear rate γ should vary. Furthermore, the shear stress τ on the inner walls of the capillary tube also varies according to the local mixing state. In other words, the viscosity μ of a mixture of polymer compounds changes with how uniform the mixing is, and if we look at the viscosity definition of the above formula, we can see that there is extreme instability in which both the denominator and the numerator vary. Put another way, there is no viscosity that governs the mixture as a system, and instead the system is in a state of constant change.
Japanese Laid-Open Patent Application 2000-143869, which is another source of disclosed information, discloses a technique of manufacturing a foam by pulverizing a plastic laminate and mixing with polypropylene, cornstarch, and calcium carbonate. With this technique, the foaming expansion ratio is about 10 times with just the resin that is admixed in the laminate paper, and the admixture of another plastic is proposed in order to increase the foaming expansion ratio. Close examination of this technique reveals that, as seen in the embodiments and examples of the invention, a foamable composition is made up of plastic from pulverized laminate paper and added calcium carbonate. Cornstarch and the paper component of plastic laminate paper are added as fillers, the components are stirred and mixed, put into the hopper of an extrusion molding machine, kneaded, heated, melted, and pressurized while water is supplied, and then discharged into the air and expanded to obtain a continuous foam. One way to increase the foaming expansion ratio is to add a plastic material that is miscible with the plastic component of the plastic laminate paper. After the plastic material, organic plant component, and inorganic component are kneaded, water is added as a foaming agent, and it is considered important that the various components be uniformly dispersed if a uniform foam is to be obtained. To this end, it is preferable if discarded plastic laminate paper is pulverized and made into pellets in a pelletizer, to which are added the required amounts of plastic, organic plant component, and inorganic component, the components are stirred and kneaded in a mixer, and the resulting composition is supplied to an extrusion molding machine. With this technique, the manufacturing process is characterized in that the method for mixing the materials involves merely mixing the plastic component, plant component (such as cornstarch or paper), and inorganic component.
Even with this disclosed technique, there is no special technical description of the above mixing step other than to say that the goal is to obtain a uniform form by merely subjecting raw materials of different melting temperatures and viscosities to a kneading step, and achieving the uniform dispersion of the various components. It is obvious that this disclosed information does not solve the problem related to viscosity and fluidity encountered by the inventors of the present invention.
A foam is basically a resource-conserving material in that the amount of materials consumed is extremely small. Foams have excellent adiabatic properties and impact resistance, and are lightweight, which means that they can be used in a wide variety of situations, including applications as packaging materials, and for this reason resin manufacturers and others have conducted intensive research into foams. The flip-side of being lightweight is that foams also have a large volume, so their development requires adequate consideration of their material characteristics as related to recyclability and disposal. As already touched upon in prior art, if a foam could be made from pulverized waste paper or another plant-derived fibrous component or a starch or the like and, if needed, a plastic resin or the like that has been confirmed to be safe to humans and the environment, this would have considerable social significance as well, in view of the high cost of developing new resins and so forth. The present invention solves the problems with viscosity and fluidity inevitably encountered with mixtures of a plurality of different types of polymer compound in the manufacture of a foam by mixing a plastic resin with the above-mentioned natural materials, and establishes a method for manufacturing a foam of uniform quality, with which stable manufacture on an industrial scale is possible.